1. Field of the Invention
The present invention relates to an image sensing device and an imaging system.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2001-230974 discloses that the greater a signal level of an optical signal output from a pixel to each of vertical output lines V1 to V3 is, the lower a voltage of each of the vertical output lines V1 to V3 is, in a solid-state image sensing device as shown in FIG. 9 of Japanese Patent Laid-Open No. 2001-230974. In this case, in the solid-state image sensing device shown in FIG. 9 of Japanese Patent Laid-Open No. 2001-230974, in a column where a signal of a pixel on which very strong light is incident is read out to a vertical output line (any one of V1 to V3), the source-drain voltage of a load MOS (corresponding one of M51 to M53) connected to the vertical output line drops to 0 V so that the load MOS is turned OFF. When the load MOSs M51 to M53 are turned OFF, the voltage drop across a GND line 4 decreases. Because of this, output voltages of dark pixels and optical black (OB) pixels are different between a row including a pixel on which strong light is incident and a row that does not include such a pixel. As a result of this, in an image including a strong spot light, an abnormality occurs in a signal level of pixels on the right and left of the spot light, so that a luminance-incremented white band output or conversely, a darkening black band output occurs. In other words, horizontal smear occurs.
To address this, Japanese Patent Laid-Open No. 2001-230974 discloses that when an optical signal is output from a pixel, the vertical output lines V1 to V3 are clipped so that the voltage thereof will not drop so as to be equal to or lower than the drain voltage that allows the load MOSs M51 to M53 to operate in a saturation region, as shown in FIG. 1 of Japanese Patent Laid-Open No. 2001-230974. Consequently, according to Japanese Patent Laid-Open No. 2001-230974, since it is possible to prevent a load MOS from being tuned OFF, an abnormality in a signal level in a row including a pixel on which strong light is incident can be reduced. Thus, horizontal smear in the obtained image can be reduced.
Japanese Patent Laid-Open No. 2005-217158 discloses that an operational amplifier 120 amplifies a signal output from a pixel to a vertical output line 106, and the amplified signal is accumulated in an accumulation capacitance 112, as shown in FIG. 3 of Japanese Patent Laid-Open No. 2005-217158. In this operational amplifier 120, a source grounded field effect transistor of a constant current circuit is connected between an input transistor 125 and a GND wiring 132 as show in FIG. 2 of Japanese Patent Laid-Open No. 2005-217158.
Japanese Patent Laid-Open No. 2008-042679 discloses that when a noise level corresponding to a pixel reset state is output from an operational amplifier 10 to a noise transmission path NT, a first clip transistor 19 clips the potential of the noise transmission path NT, as shown in FIG. 3 of Japanese Patent Laid-Open No. 2008-042679. The drain of the first clip transistor 19 is connected to an output terminal of the operational amplifier 10, and its source is connected to a capacitance 14 for holding a noise level via a transmission transistor 12. Consequently, according to Japanese Patent Laid-Open No. 2008-042679, a voltage supplied from the operational amplifier 10 via the noise transmission path NT to and accumulated in the capacitance 14 is kept from being higher than an original reset level, thus enabling suppression of darkening.
On the other hand, in an image sensing device in which a differential amplifier in each column amplifies a signal output from a pixel in each column in a pixel array to a vertical output line and outputs the amplified signal to an output amplifier, a horizontal smear in an image corresponding to an image signal cannot be sufficiently suppressed when some of the pixels are irradiated with strong light.
For example, when some of the pixels are irradiated with strong light, an optical signal whose signal level is excessive is output from the some pixels to the vertical output lines. At this time, even though a MOS transistor is going to clip a potential of each of the vertical output lines, an optical signal whose signal level is excessive flows in the differential amplifier until the MOS transistor is turned ON. In this case, when the differential amplifier amplifies an optical signal, a constant current circuit included in the differential amplifier supplies an excessive current to a ground line. Due to an influence of this, it is impossible for a differential amplifier in another column to output an appropriate signal. Accordingly, a horizontal smear occurs in an image obtained corresponding to an image signal output from the image sensing device.